Polyolefin crystallinity/morphology

Polyethylene/PolypropyleneBlends Blends of polyolefins represent one of the most studied polymer systems in the field of immiscible blends. Studies on blends of iPP with crystallizable and/or amorphous polyolefins, such as polyethylenes (PEs) of various density (HD PE, ED PE, LLDPE), atactic PP, propylene-etylene elastomers, and polyisobutylene, have shown that their properties depend strongly on the crystalline morphology, crystallization conditions, and composition... 

In this work the suitabihty of the Modified Von Mises criterion to describe the yielding behavior of toughmed propylene polymers was investigated. Two polypropylene homopolymeis with different crystalline morphology, two different propylene copolymers and three polypropylene mechanical blends containing 10, 20 an 30%wt of an elastomeric polyolefin were included in this study. 

A preliminary stndy on the viscoelastic behaviour of polyolefin foam sheets with different chemical (PE and PP) and cellular structure by DMA, in the low freqnency and low compression ranges, is presented. DSC and SEM are also used to determine the morphological parameters of the samples. A connection between the morphological properties (apparent degree of crystallinity), type of cellular structure, homogeneity, cell size and shape, cell wall thickness) and the viscoelastic behavionr, a basic key for the development of mechanical and insnlating applications, has been established. 9 refs. 

In order to inhibit the oxidation of polymers, the antioxidant has to be present in sufficient concentration at the various oxidation sites. In this respect, both the distribution of antioxidants and the morphology of the host polymer assume greater significance. Examination of the distribution of photo-antioxidants in typical commercial semi-crystalline polymers, such as polyolefins, has shown " " " that they are rejected into the amorphous region on the boundaries of spherulites. Such nonuniform distribution of antioxidants leads to an increase in their concentration in the amorphous region, which is most susceptible to oxidation (the crystalline phase is normally impermeable to oxygen). However, in the case of polymer blends, a nonuniform distribution of antioxidants can undermine the overall stability of the blend, especially when the more oxidizable component of the polymer blend is left unprotected. 

It is worthwhile to note that chemistry plays a major role in the morphology and control of mechanical properties in complex systems like PPE blends with crystalline polymers, such as polyolefins, polyamides (PA) and polyesters (18). The amount of copolymer formed during the reactive extrusion between functionalised PPE and PA has a significant effect on the impact-strength of blends. The latter levels off only above 10% of copolymer. 

Isotactic PP and ICP can be physically blended with EPR to adjust the modulus and the impact resistance. Recently, new types of polyolefin copolymers have been developed to replace EPR with higher compatibility with PP using metallocene catalysts. These polyolefin copolymers will be discussed in Section 8.4. Nitta et al. (7) compared the PP/EPR blends with different EPR in terms of My, MWD, and crystallinity. The different compatibility of EPR with PP creates different morphologies, which are shown in Fig. 8.5. 

The miscibility of olefin copolymers such as ethylene-a-olefin copolymers was found to be controlled by the structural composition and the primary strucmre of the copolymers. Using these copolymers, binary blends with various compatibilities were prepared and the effects of compatibihty on mechanical properties in the binary blends were investigated. The tensile properties in binary blends of iPP with rubbery olefin copolymers are considerably influenced by the miscibility between iPP and the copolymers. The miscibility of iPP with other polyolefins is described in detail based on the dynamic mechanical properties, morphology observation, and solidification process. It is found that EBR, EHR, and EOR having more than 50 mol% of a-olefin are miscible with iPP in the molten state. In the solid state, the miscible copolymers are dissolved in the amorphous region of iPP, although the copolymers are excluded from crystalhne lattice of iPP. The isotactic propylene sequence in the EP copolymers with a propylene-unit content of more than 84 mol% participates in the crystallization process of iPP, resulting that a part of the EP copolymers is included in the crystalline lattice of iPP. 

Like binary blends, PA6/(g-PE/g-EPDM) composites are two-phased. Irrespective of the components ratio, their individual effects of melting and crystallization in blends are shown on the DSC curve. The comparative analysis of structure-morphology features for ternary blends, PA6/(g-PE/g-EPDM), and binary blends indicated that g-EPDM added to PA6/g-PE affects crystallization of the polyolefin as well as polyamide components. The results of DSC on the two types of blends, PA6/ (g-LDPE/g-EPDM) and PA6/(g-HDPE/g-EPDM), show increased crystallinity in g-PE against a binary blend (108,109). This effect results, most likely, from easier crystallization of g-HDPE in the presence of g-EPDM owing to raised molecular mobility because of plasticization by the elastomeric phase. 

The properties of polyolefin/starch blends depend on starch content, degree of dispersion in the polymer matrix, on sample morphology, interactions between components, degree of crystallinity, presence of structural defects as well as preparation or processing conditions. Starch was initially added into polymers as a filler to decrease the price of products. Typical examples are blends of starch with PE, PVC and mbber. Since the 1990s, starch has been blended with conventional polymers to facilitate biodegradation of polymers, in particular polyolefins. 

Irradiation changes the chemical structure of polyolefins, especially in the amorphous phase, while the crystalline phase is more resistant to its action. As mentioned earlier, all polyolefin/El or polyolefin/TPE irradiated blends can be classified in three different morphologies depending on the balance between chain scission and cross-linking. 

The photostabiUty of different types of polyolefins varies depending on their chemical structures, crystallinity, and morphology. Polypropylene (PP) is especially sensitive to UV radiation and must be stabilized against the effects of solar... 

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